1. Technical Field
This invention relates to novel apparatus and process for filtering ultra pure water. In one aspect, the invention relates to a separations process for providing particle-free ultra pure water for use in the electronics and semiconductor industries.
2. Background
High purity water is required for semiconductor device manufacturing. In semiconductor manufacturing, high purity water is used primarily for wafer rinsing. This rinsing process removes both contaminants inadvertently introduced to the wafer during the manufacturing process as well as chemicals intentionally introduced to the wafer for the purpose either of cleaning or of etching patterns into the semiconductor surface or the conductive or dielectric thin films grown or deposited on this surface as required to produce a semiconductor device.
These chemicals include, but are not limited to, hydrofluoric acid, ammonium fluoride, phosphoric acid, piranha solutions (the generic name used for mixtures of sulfuric acid and a strong oxidizer: usually hydrogen peroxide or ozone), and the two component solutions comprising the industry standard RCA cleaning method, i.e, the so-called SC-1 and SC-2 solutions. (W. Kern and D. A. Puotinen, RCA Review, Volume 31, pp. 187, 1970.)
SC-1 is a mixture of ammonium hydroxide, unstabilized hydrogen peroxide, and ultra pure water. SC-2 is a mixture of hydrochloric acid, unstabilized hydrogen peroxide, and ultra pure water. Ultra pure water rinses are also used to remove the solvents used for the removal of photoresists. Ultra pure water is the chemical used in the greatest quantity in semiconductor manufacturing, and extremely large quantities of ultra pure water are produced in modern semiconductor production facilities.
The greatest cause of yield loss in modern semiconductor manufacturing is particulate contamination. Particles interrupt the photolithographic patterns that define integrated circuit components and can cause open circuits in the conductive metal thin films that wire these circuits together. When attached to a semiconductor wafer, particles are extremely difficult to remove as they electrostatically bond to the wafer. Therefore, all of the fluids which contact the semiconductor wafer must be rigorously filtered to remove particles. This includes ultra pure water.
Preferred characteristics for an ultra pure water filter, in addition to excellent particle removal efficiency, include quick rinse up, no shedding of its materials of construction into the water stream, no contribution of ionic species, minimal introduction of TOC (Total Oxidizable Carbon or Total Organic Carbon) to the water stream, and ozone and hot ultra pure water (UPW) compatibility. Ozone compatibility is important because ozone enables the destruction of microbial matter without the introduction of additional chemical agents such as chlorine or hydrogen peroxide which are themselves considered contaminants in ultra pure water. Microbial matter, such as bacteria and viruses, is the primary source of TOC.
Ozone is produced by spark discharge from ultra pure oxygen and eventually decomposes into diatomic oxygen in ultra pure water. It therefore offers an extremely clean method of system sterilization. This ability of ozone to kill biological contaminants is especially critical for ultra pure water systems as bacteria and microorganisms can grow rapidly in the conditions commonly found in ultra pure water distribution loops.
The above-described characteristics are desirable for all of the various types of filters typically used in ultra pure water systems: microfilters, tangential-flow or cross-flow filters such as ultra filters, and RO (reverse-osmosis) membranes.
Ozone can be utilized in an ultra pure water system in various ways. It can be used for initial sterilization of a system after construction. It can be used periodically afterward for re-sterilization after system component replacement or maintenance, or it can be used continuously.
Hot ultra pure water (UPW) has become desirable for semiconductor wafer rinsing as the diffusive removal of chemicals from solids is accelerated by heat, the diffusion process being temperature dependent. Additionally, most substances (although not all) are more readily soluble in hot water than in cold. The superior cleansing properties of hot UPW are perceived to extend as well to the removal of particles from the wafer surfaces. Currently used ultrafilters are incompatible with hot UPW. (M. S. Homick, K. L. Fulford, and S. Browne, "Design and Performance of a Central Hot Ultrapure Water System", Proceedings of the 1992 Microcontamination Conference, pp. 753-763, 1992.) The ceramic membrane ultrafilter is unaffected by hot UPW with or without ozone injection.
The different components used in UPW (ultra pure water) systems all have different levels of compatibility with ozone and hot UPW. Most final filters used to produce ultra pure water are not compatible with ozone. Therefore, most conventional final filters used to produce ultra pure water today must be taken out, or "valved out," of the loop during ozone sterilization. These conventional filters typically are sterilized separately with hot UPW or chemical solutions. Addressing final filters separately during frantic plant shut-downs creates extra work, thus increasing the length and cost of the shut-down. In many cases, ozone sterilization of the service loop will take place without the final filters on line. This situation invites particulate contamination of the service loop. Alternatively, hot ultra pure water sterilization, which is ideal from the standpoint of cleanliness, requires significant extra effort and equipment during the filter sterilization, and hot UPW may also attack the materials of construction of these final filters. Commonly used chemical oxidizing agents, such as hydrogen peroxide, have undesirable drawbacks associated with contamination and rinse up problems.
A preferred filter would also be resistant to particle release because of pressure fluctuations. Filter design is an important factor in determining the particle shedding characteristics of ultrafilters in both normal and pulsed conditions. Many types of filters including ultrafilters will shed particles or pass trapped particles when the feed pressure pulsates. (Meltzler, T. H., "An Investigation of Membrane Cartridge Shedding: A Quantitative Comparison of Four Competitive Filters", Transcripts of Sixth Annual Semiconductor Pure Water Conference, pp. 221-239, Santa Clara, Calif., Jan. 15-16, 1987.)
Conventional processes for producing ultra pure water typically use dead-end filtration with polymeric membranes. For example, conventional ultrafilters (similar in design to RO membrane systems) are used to produce high purity water for electronics applications. Spiral wound membranes, another commonly employed technology, require low solids in the feed stream and have high pressure drops associated with the restrictive liquid flow paths through their extremely narrow channels. The fouling of polymeric ultrafilters is a problem as is the requirement to periodically clean these filters off line. (D. Sinha, "Pretreatment Process Considerations for the Semiconductor Industry", Ultrapure Water 7(6), pp. 21-30 (1990).)
These and other known processes for producing ultra pure water for the electronics or semiconductor manufacturing applications, as represented in the following.
U.S. Pat. No. 5,292,439, assigned to Mitsubishi Kasei of Japan, discloses a method for preparing ultra pure water for use in electronics industries. The patent discloses that the water quality of ultra pure water is largely affected by impurities eluted from the materials constituting the purification apparatus. (Col. 1, bottom). An improved ion exchange process is provided to overcome the problems of the impurities eluted from the materials constituting the purification apparatus.
U.S. Pat. No. 5,259,972, assigned to Nippon Rensui and Mitsubishi Denki of Japan, discloses apparatus and method for purifying water for use in the electronics industry. The patent discloses that it is known to combine an ultrafilter membrane with ion exchange, ultraviolet (UV) sterilizers, cartridge filters, reverse osmosis (RO) and other components to produce ultra pure water. (Col. 1, line 18).
U.S. Pat. No. 5,160,429, assigned to Tadahiro Ohmi of Japan, discloses supplying ultra pure water for the electronic industry. The patent shows the use of ultrafiltration units 108 and 109.
U.S. Pat. No. 5,128,043 discloses apparatus and method for purifying water for use in the electronic industry. The patent discloses using an electric field. A specific object is to remove particles. Another object is to provide a method and apparatus not affected by ozone. A membrane filter 4 removes particulates. A final filter 12 is sized at 0.2 microns.
U.S. Pat. No. 5,124,033, assigned to Shinko Pantec of Japan, discloses an ultra pure water producing system for the semiconductor manufacturing process. Metallic ions and particulates are removed. An ultrafilter 39 is disclosed. The patent discloses that when water comes in contact with the materials of the devices and pipings, materials such as silica are leached out.
U.S. Pat. No. 5,073,268, assigned to Ebara Corp. of Japan, discloses a process for purifying water or ultra pure water for use in the electronics industry. An ultrafilter unit 22 is disclosed.
U.S. Pat. No. 5,061,374, assigned to Micron Technology of Boise, Idaho, discloses an ultra pure water system employing a second train which reduces particulate in the manufacture of integrated chips (IC's). The patent discloses ozone and membrane filters and reduces particulate in the water to a level of less than 30 counts per milliliter sizes 0.1 micron to 1 micron. (Col. 1, lines 48-50.)
U.S. Pat. No. 5,024,766 discloses water purification which reduces particulate in semiconductor manufacturing. The patent discloses ozone and submicron filter 20. Polymeric membrane filters marketed as Fluorodyne are disclosed at the top of Col. 9.
U.S. Pat. No. 4,879,041, assigned to Hitachi of Japan, discloses a process for producing ultra pure water for semiconductor manufacturing. The patent discloses ozone and membrane filters.
U.S. Pat. No. 4,863,608, assigned to Nomura Micro Science of Japan, discloses a process for producing ultra pure water for integrated circuits manufacturing. The patent discloses ultrafiltration and microfiltration.
There are many costly problems and drawbacks associated with the utilization of conventional loop or final filters in the conventional or prior art processes or methods for producing ultra pure water. One problem involves using ozone sterilization and/or high temperatures. If ozone is present, none of the conventional polymeric membranes are resistant with the exception of polytetrafluoroethylene (PTFE). If high temperature operation is required, organic extractables and short membrane life become problems for many polymeric membranes. Ozone can cause the oxidation and therefore growth of a polymeric filter's pore size resulting in lower particle retention levels with increasing exposure to ozone.
Accordingly, a drawback of conventional processes is the failure of the filter media surfaces or opening of the filter pores because of oxidation of non-resistant organic materials in the presence of strong oxidizing agents such as ozone.
Another drawback is the failure of welded polymer seams along the filter pleat.
A further drawback is the requirement of high flux or throughput for economic viability.
Another drawback is that ozone-resistant organic materials suffer from low flux.
The conventional methods utilize an organic copolymer membrane. Therefore, another drawback in dealing with such materials is that the oxidizing environment will degrade or produce organic impurities which raise TOC levels.
It is also a drawback of conventional methods in the requirement for a separate sterilizing agent because of non-compatibility with ozone, which is the current sterilizing agent of choice in the electronics industry.
Therefore, it is an object of the present invention to provide a stable, high flux ultrafiltration membrane for filtering ultra pure water.
It is another object of the present invention to provide a process for filtering particles from ultra pure water for use in electronics manufacturing.
It is a further object of the present invention to provide a process for filtering ultra pure water through an ultrafilter in hot, ozonated high purity water with no detectable metal extractables.
It is still a further object of the present invention to provide a process for filtering ultra pure water through a filter which passes toxicology tests to provide for use in food and pharmaceutical applications.
It is still a further object of the present invention to provide a process for filtering ultra pure water through a filter with no particle shedding.
It is still a further object of the present invention to provide a process for filtering ultra pure water through a filter which provides long membrane life and an economical and compact design.
It is still a further object of the present invention to provide a process for filtering ultra pure water through a filter providing a metal oxide membrane on a ceramic support which is able to withstand prolonged contact with ozone and/or high temperatures.
It is another object of the present invention to provide a process for filtering ultra pure water through a filter providing a metal oxide membrane on a ceramic support which does not alter ultra pure water properties, e.g., without leaching ions into the ultra pure water.
It is another object of the present invention to provide a process for filtering ultra pure water through a filter providing a metal oxide membrane on a ceramic support which maintains acceptable ultra purity filtration without leaching ions into the ultra pure water produced over high recoveries per pass, which result in longer times of water contact with the filter.
It is a further object of the present invention to provide a process for filtering ultra pure water through a filter which can be maintained and returned to operable condition by sterilizing with ozone or cleaning with steam, acid, and/or basic solutions.
A further object of the present invention is to provide a process for filtering ultra pure water through a filter which is integrity testable and bacteria and pyrogen retentive while providing acceptable flux and permeability, i.e., flux per unit transmembrane pressure, over a wide range of operating conditions.
It is yet another object of the present invention to provide a membrane filter which is insensitive to sudden process upsets such as high pressure pulses or temperature or both.
Another further object of the present invention is to provide a filter which can be back-pulsed with clean water flux to dislodge and remove contaminated trapped by the membrane layer.
These and other objects of the present invention will be described in the detailed description of the invention which follows. These and other objects of the present invention will become apparent to those skilled in the art from a careful review of the detailed description and from reference to the figures of the drawings.